Metallic medical device parts that are suitable for surgical use and/or implantation in a patient can be difficult to create. Typically, the parts are formed using machining techniques that physically shape and finish the part surfaces. However, when parts reach a small enough scale (e.g., having a major axis below about 0.3 inches and/or a minor axis below about 0.08 inches), the cost of meeting the tolerances required to form such parts with conventional machining can become prohibitively expensive.
Casting provides a potential cost effective, alternative technique for forming such small-scale parts. The casting of medical-grade metals in a molten state for forming such parts, however, presents a number of challenges. In general, metallic materials that are suitable for medical applications are difficult to cast into small-scale pieces owing to their high chemical reactivity at temperatures close to the material's high melting point or range. In particular, as these molten metals are heated higher and higher above their melting point or range, they tend to become more and more reactive (e.g., undergoing oxidation reactions or other unwanted reactions with the mold surface). Such reactions lead to the formation of impurities that contaminate the metal parts, which result in various detrimental consequences. The presence of impurities skews the composition of the metal such that it may not meet the desired standard of a medical-grade material, thereby disallowing the use of the cast piece for the intended application. As well, the presence of the impurities can detrimentally affect the mechanical properties of the metallic material (e.g., lowering the strength of the material). Furthermore, such reactions can lead to surface texturing, which results in substantial, undesirable roughness on the surface of the cast piece. For example, using the surface roughness value Ra, as known in the art for characterizing surface roughness, cast pieces utilizing stainless steel alloys and/or titanium alloys are typically exhibit an Ra value between about 100 and 200 under good working conditions. Indeed, the production of small-scale cast pieces with such materials can be very difficult since the scale of the roughness features approaches the scale of the individual piece. These detrimental effects drive one to use lower temperatures for filling molds. If the temperature of the molten metal is not heated enough, however, the casting material can cool too quickly, leading to incomplete filling of the cast mold.
Accordingly, a need exists for improved techniques of casting small-scale metal pieces such that appropriately sized medical parts can be cast for use in surgical and implantation applications.